Macroscopic and Microscopic Properties of Cement Paste with Carbon Dioxide Curing

Carbon dioxide is the main component of greenhouse gases, which are responsible for an increase in global temperature. The utilization of carbon dioxide in cement-based materials is an effective way to capture this gas. In this paper, the influence of carbon dioxide curing on the setting time, the electrical resistivity, dry shrinkage ratio, water absorption by unit area and mechanical strengths (flexural and compressive strengths) were determined. The scanning electron microscope, X-ray diffraction and thermogravimetric analysis were obtained to investigate the mechanism of carbonation reaction of cement paste. Water–cement ratios of cement paste were selected to be 0.3, 0.4 and 0.5. Results showed that carbon dioxide curing could accelerate the setting of cement paste. The electrical resistivity decreased with the increasing water–cement ratio and increased with the carbon dioxide curing. Moreover, the evaluation function for the curing age and dry shrinkage rate or the mechanical strengths fit well with the positive correlation quadratic function. The water absorption by unit area increased linearly with the testing time. The carbon dioxide curing led to increasing the mechanical strengths and the dry shrinkage ratio. Meanwhile, the carbon dioxide curing demonstrated a decreasing effect on the water absorption by unit area. The mechanical strengths were improved by the carbon dioxide curing and increased in the form of quadratic function with the curing age. As obtained from the microscopic findings, that the carbon dioxide curing could accelerate the reaction of cement and improve the compactness of cement paste.


Introduction
Carbon dioxide is the main culprit of the greenhouse effect. A large amount of carbon dioxide gas will damage the ecological environment. Methods for treating carbon dioxide including plant absorption method, ocean absorption method, agricultural consumption and mechanical capture method. These carbon dioxide treatment methods possessed high cost, low efficiency or poor feasibility [1,2]. In consideration of these reasons, new methods of treating carbon dioxide were needed.
Cement concrete is the most extensive artificial building materials which has been widely used in the construction industry [3,4]. During the production of cement, approximately 5-7% of global anthropogenic carbon dioxide has been emitted by the combustion of fossil fuels and the decomposition of limestone during the production of cement clinker [5,6]. Additionally, other emissions of carbon dioxide induced damage to the ecological environment. As reported in Ravikumar's research, the utilization of carbon dioxide in cement-based materials is an effective way to capture this gas [7]. The carbon dioxide can react with cement clinker and form calcium carbonate, thus, making the concrete

Specimen Preparation
The specimens were prepared following these steps: The cement was poured into the water slowly in the NJ-160 cement mixer produced by Shangyu Yueda Instrument Factory, Shaoxing, China and mixed for 2 min with the stirring speed of 140 rpm. Then, the onther stirring speed of 285 rpm and stirring time of 2 min was provided for mixing the fresh samples. After the mixing was finished, the fresh paste was poured into the molds for manufacturing the specimens with sizes of 50 mm × 50 mm × 50 mm and 40 mm × 40 mm × 160 mm. CCB-70B carbonation tank produced by Suzhou Donghua Test Instrument Co., Ltd., Suzhou, China, was offered for the carbon dioxide curing of cement paste. The concentration of carbon dioxide was 8% by total mass of gas. In this study, the curing environment with 95% relative humidity and 20 • C was the standard curing environment. Table 3 demonstrates the mixing proportion of the specimens in this experiment.

The Setting Time Experiment
The testing mold for the setting time of cement paste was circular truncated cone with the top diameter of 60 mm, bottom diameter of 70 mm, and a height of 40 mm. Cement standard consistency setting time tester (standard favicat tester) cement Vicat tester manufactured by Hebei Yiwei Trading Co., Ltd., Cangzhou, China was applied in the determination of the setting time of cement. The experimental process was carried out according to GBT1346-2011 [25].

The Electrical Resistivity Experiment
The electrical resistivity was determined by two electrode method. Two pieces of 304 stainless steel mesh with the square hole sizes of 4.75 mm served as two electrode. TH2810D LCR digital electric bridge (Changzhou Tonghui Co., Ltd., Changzhou, China) was provided for the measurement of AC electrical resistivity. The testing frequency of TH2810D LCR digital electric bridge was 10,000 Hz, meanwhile, the testing voltage was 1 V. The electrical resistivity can be calculated by Equation (1): where R means the electrical resistance of specimen, S is the interface area of specimen and L is the length of specimen. The measurement of AC electrical resistance was shown in Figure 1. The testing method of the electrical parameters in this section referred to refs. [23,26].

The Experiments of Mechanical Performance and Permeability
The YAW-300 microcomputer (Hengruijuan Co., Ltd., Jinan, China) full-au universal was used for testing the compressive and flexural strengths of specime loading speeds for compressive and flexural strengths were 2.4 kN/s and 0.05 k spectively. The experiment of mechanical strengths was conducted according t 17671-1999 Chinese standard [28]. The shrinkage rod of the dial indicator manuf by Kaiyue Co., Ltd., Cangzhou, China supporting the middle of one end of the gular specimen was applied in the determination of dry shrinkage rate. When the of the specimen changes, the dial indicator reads out the value of the length c Through this method, the dry shrinkage rate was measured. The measurement shrinkage rate is shown in Figure 2. The measuring environments for the shrinka were standard curing environment (95% relative humidity and 20 °C) and carbo ide curing environment (8% by total mass of gas, 65% relative humidity and 20 °C Cylinder specimens with size of Φ100 mm × 50 mm were used for the measu of water absorption by unit area. Before testing, all specimens were desiccated vacuum drying oven provided by Shanghai Hecheng Instrument Manufacturi Ltd., Shanghai, China, with a continuous temperature of 60 °C for 4 days. Af process, the lateral sizes of specimens were sealed with epoxy resin to preven adsorption. When the epoxy resin was hardened, the bottom surface of each sp was immersed in water with the depth of 2 cm. The water absorption by unit area expressed by Equation (2) as follows: where Vw (g/m 2 ) is the water absorption by unit area, S (g/(m 2 /min 0.5 )) is the sorpt the material, t (min) is the elapsed time and b (g/m 2 ) is the initial water absorptio curing condition of all samples is illustrated in Table 4.

The Experiments of Mechanical Performance and Permeability
The YAW-300 microcomputer (Hengruijuan Co., Ltd., Jinan, China) full-automatic universal was used for testing the compressive and flexural strengths of specimens. The loading speeds for compressive and flexural strengths were 2.4 kN/s and 0.05 kN/s respectively. The experiment of mechanical strengths was conducted according to GB/T 17671-1999 Chinese standard [27]. The shrinkage rod of the dial indicator manufactured by Kaiyue Co., Ltd., Cangzhou, China supporting the middle of one end of the rectangular specimen was applied in the determination of dry shrinkage rate. When the length of the specimen changes, the dial indicator reads out the value of the length change. Through this method, the dry shrinkage rate was measured. The measurement of dry shrinkage rate is shown in Figure 2. The measuring environments for the shrinkage rate were standard curing environment (95% relative humidity and 20 • C) and carbon dioxide curing environment (8% by total mass of gas, 65% relative humidity and 20 • C).

The Experiments of Mechanical Performance and Permeability
The YAW-300 microcomputer (Hengruijuan Co., Ltd., Jinan, China) full-automatic universal was used for testing the compressive and flexural strengths of specimens. The loading speeds for compressive and flexural strengths were 2.4 kN/s and 0.05 kN/s respectively. The experiment of mechanical strengths was conducted according to GB/T 17671-1999 Chinese standard [28]. The shrinkage rod of the dial indicator manufactured by Kaiyue Co., Ltd., Cangzhou, China supporting the middle of one end of the rectangular specimen was applied in the determination of dry shrinkage rate. When the length of the specimen changes, the dial indicator reads out the value of the length change. Through this method, the dry shrinkage rate was measured. The measurement of dry shrinkage rate is shown in Figure 2. The measuring environments for the shrinkage rate were standard curing environment (95% relative humidity and 20 °C) and carbon dioxide curing environment (8% by total mass of gas, 65% relative humidity and 20 °C). Cylinder specimens with size of Φ100 mm × 50 mm were used for the measurement of water absorption by unit area. Before testing, all specimens were desiccated in the vacuum drying oven provided by Shanghai Hecheng Instrument Manufacturing Co., Ltd., Shanghai, China, with a continuous temperature of 60 °C for 4 days. After this process, the lateral sizes of specimens were sealed with epoxy resin to prevent water adsorption. When the epoxy resin was hardened, the bottom surface of each specimen was immersed in water with the depth of 2 cm. The water absorption by unit area can be expressed by Equation (2) as follows: where Vw (g/m 2 ) is the water absorption by unit area, S (g/(m 2 /min 0.5 )) is the sorptivity of the material, t (min) is the elapsed time and b (g/m 2 ) is the initial water absorption. The curing condition of all samples is illustrated in Table 4.

Types
The  Cylinder specimens with size of Φ100 mm × 50 mm were used for the measurement of water absorption by unit area. Before testing, all specimens were desiccated in the vacuum drying oven provided by Shanghai Hecheng Instrument Manufacturing Co., Ltd., Shanghai, China, with a continuous temperature of 60 • C for 4 days. After this process, the lateral sizes of specimens were sealed with epoxy resin to prevent water adsorption. When the epoxy resin was hardened, the bottom surface of each specimen was immersed in water with the depth of 2 cm. The water absorption by unit area can be expressed by Equation (2) as follows: where V w (g/m 2 ) is the water absorption by unit area, S (g/(m 2 /min 0.5 )) is the sorptivity of the material, t (min) is the elapsed time and b (g/m 2 ) is the initial water absorption. The curing condition of all samples is illustrated in Table 4. In order to obtain the thermal analysis curves, these steps can be described as follows: The samples cured in carbon dioxide curing environment for 1 day, 3 days and 28 days were taken and immersed in the absolute ethanol for 4 days to prevent the hydration of cement. After that, all samples were dried in the vacuum drying oven at the temperature of 60 • C for 4 days before the measurement. The soybean size of hardened cement paste was taken from the inner specimens cured in the carbon dioxide curing environment for 1 day, 3 days and 28 days. The dried samples were sprayed by gold before measurement. Some other samples were filtrated with a 74 µm sieve before thermogravimetric experiment.
The weighed sample powder was placed in an alumina pan of confined space in the thermogravimetric analyzer. The nitrogen atmosphere, whose flow rate is 20 mL/min, was provided as shielding gas. The temperature in the thermogravimetric analyzer ranged from 20 • C to 950 • C. The experimental process of the thermal analysis curves is referred in Refs [24,28]. The soybean size of hardened cement paste sprayed by gold was used for the measurement of scanning electron microscope (SEM). JSM-6360LV scanning electron microscope (Japan electron optics laboratory, Tokyo, Japan) and TGA 4000 thermogravimetric analyzer provided by Perkin Elmer Instrument Co., Ltd., New York, NY, USA were applied in the measurement of thermal analysis curves and SEM photos.  Table 5 shows the fitting result of the relationship between the increasing rate of the setting time and the water-cement ratio of cement paste. The setting time includes initial setting (IS) time and final setting (FS) time. As obtained from Figures 3 and 4 and Table 5, the setting time increased with the increasing water-cement ratio and the increasing rate increased linearly when the specimens were cured in a standard curing environment. This was attributed to the fact that cement paste with higher water-cement ratio exhibited more free water leading eventually to delay the setting time [29,30]. However, when the specimens were cured in the CO 2 curing environment, the setting time decreased, with the decreasing rate corresponding to the linear function with the increasing water-cement ratio due to the fact that the CO 2 curing accelerate the carbonation reaction leading to decreasing the setting of cement [31,32]. Meanwhile, when CO 2 curing was provided, the setting time of cement paste with a higher water-cement ratio was lower due to the fact that the carbonation speed of cement paste with a higher water-cement ratio was faster [33,34]. It can be acquired from Table 5, the increasing rate of setting time conformed to the linear function with the water-cement ratio. The fitting degrees were higher than 0.98, which confirmed the accuracy of the fitting function. conformed to the linear function with the water-cement ratio. The fitting degrees were higher than 0.98, which confirmed the accuracy of the fitting function.

Equation
Types  Figure 5 shows the electrical resistivity of cement paste cured in standard curing environment and CO2 curing environment, respectively. As observed from Figure 5, the electrical resistivity increased in the form of cubic function with the curing time. This was due to the fact that the free water in the pore solution was consumed with the development of cement hydration [36,37]. Therefore, the conductivity was reduced and the electrical resistivity increased with the curing age. Meanwhile, the CO2 curing on cement paste could increase the content of calcium carbonate and a dense structure was formed on the hydration products of cement, thus, hindering the migration of conductive ions and increasing the electrical resistivity of cement paste [38,39]. When the specimens were cured in standard curing environment, the electrical resistivity decreased with the increasing water-cement ratio. This was attributed to the fact that free water increased with     Figure 5 shows the electrical resistivity of cement paste cured in standard curing environment and CO2 curing environment, respectively. As observed from Figure 5, the electrical resistivity increased in the form of cubic function with the curing time. This was due to the fact that the free water in the pore solution was consumed with the development of cement hydration [36,37]. Therefore, the conductivity was reduced and the electrical resistivity increased with the curing age. Meanwhile, the CO2 curing on cement paste could increase the content of calcium carbonate and a dense structure was formed on the hydration products of cement, thus, hindering the migration of conductive ions and increasing the electrical resistivity of cement paste [38,39]. When the specimens were cured in standard curing environment, the electrical resistivity decreased with the in-   Figure 5 shows the electrical resistivity of cement paste cured in standard curing environment and CO 2 curing environment, respectively. As observed from Figure 5, the electrical resistivity increased in the form of cubic function with the curing time. This was due to the fact that the free water in the pore solution was consumed with the development of cement hydration [35,36]. Therefore, the conductivity was reduced and the electrical resistivity increased with the curing age. Meanwhile, the CO 2 curing on cement paste could increase the content of calcium carbonate and a dense structure was formed on the hydration products of cement, thus, hindering the migration of conductive ions and increasing the electrical resistivity of cement paste [37,38]. When the specimens were cured in standard curing environment, the electrical resistivity decreased with the increasing water-cement ratio. This was attributed to the fact that free water increased with the increasing water-cement ratio, thus, improving the electrical conduction and decreasing the electrical resistivity [39,40]. However, as shown in Figure 5, the electrical resistivity increased with the increasing water-cement ratio. This could be ascribed to the fact that the rate of carbonation increased with the increasing water-cement ratio, thus, forming more calcium carbonate and preventing the migration of conductive ions leading eventually to increasing the electrical resistivity of cement paste [41,42]. It can be acquired from the result that the CO 2 curing on cement paste led to decreasing the electrical conduction and increasing the electrical resistivity of cement paste. Table 6 exhibits the values of fitting parameters of all curves. As depicted in Table 6, the fitting degrees of all the curves were higher than 0.94, which proved the accuracy of the fitting equations.

The Electrical Resistivity
Materials 2022, 15,1578 the increasing water-cement ratio, thus, improving the electrical conduction a creasing the electrical resistivity [40,41]. However, as shown in Figure 5, the electr sistivity increased with the increasing water-cement ratio. This could be ascribed fact that the rate of carbonation increased with the increasing water-cement ratio forming more calcium carbonate and preventing the migration of conductive ion ing eventually to increasing the electrical resistivity of cement paste [42,43]. It acquired from the result that the CO2 curing on cement paste led to decreasing th trical conduction and increasing the electrical resistivity of cement paste. Table 6 e the values of fitting parameters of all curves. As depicted in Table 6, the fitting deg all the curves were higher than 0.94, which proved the accuracy of the fitting equa

The Results of Mechanical Performance and Permeability
In this section, the specimens were cured in CO2 for a different amount of tim curing, they were moved to the standard curing environment for different curin Figure 6 shows the dry shrinkage rate of specimens at different curing ages. As sh Figure 6, the dry shrinkage ratio of specimens increased rapidly with the curi ranging from 1 day to 28 days, due to the fact that the free water decreased with t provement of hydration degree [44,45]. Therefore, the vacancies increased with t provement of hydration, leading to increasing the dry shrinkage rate. However, wh curing age increased from 28 days to 120 days, the dry shrinkage ratio increased with the increasing curing age. Table 7 shows the fitting results of the relationsh tween the curing age and dry shrinkage rate. It can be obtained from Table 7, th tionships between the curing age and the dry shrinkage rate satisfied the quadrati tion. Moreover, the fitting degrees of all curves were higher than 0.8, signifying c

The Results of Mechanical Performance and Permeability
In this section, the specimens were cured in CO 2 for a different amount of time; after curing, they were moved to the standard curing environment for different curing ages. Figure 6 shows the dry shrinkage rate of specimens at different curing ages. As shown in Figure 6, the dry shrinkage ratio of specimens increased rapidly with the curing age ranging from 1 day to 28 days, due to the fact that the free water decreased with the improvement of hydration degree [43,44]. Therefore, the vacancies increased with the improvement of hydration, leading to increasing the dry shrinkage rate. However, when the curing age increased from 28 days to 120 days, the dry shrinkage ratio increased slowly with the increasing curing age. Table 7 shows the fitting results of the relationship between the curing age and dry shrinkage rate. It can be obtained from Table 7, the relationships between the curing age and the dry shrinkage rate satisfied the quadratic function. Moreover, the fitting degrees of all curves were higher than 0.8, signifying correctness of fitting equations. As depicted in Figure 6, higher water-cement ratio led to decreasing the dry shrinkage ratio of specimens when cured in standard curing environment. This was attributed to the fact that the loss rate of free water was decreased by the increasing water-cement ratio thus decreasing the dry shrinkage ratio of specimens [45,46]. Furthermore, the CO 2 curing could increase the dry shrinkage ratio of specimens, due to the fact that the carbonation will consume free water inside the cement paste, resulting in increasing the shrinkage ratio. When the specimens were cured in CO 2 , a higher water-cement ratio led to the increased shrinkage ratio due to the improvement of carbonation degree [47]. The values of fitting parameters for all fitting curves are displayed in Table 7. The fitting degrees were higher than 0.8, which means that the fitting equations for all curves were reasonable.
Materials 2022, 15, 1578 8 due to the fact that the carbonation will consume free water inside the cement paste sulting in increasing the shrinkage ratio. When the specimens were cured in CO higher water-cement ratio led to the increased shrinkage ratio due to the improveme carbonation degree [48]. The values of fitting parameters for all fitting curves are played in Table 7. The fitting degrees were higher than 0.8, which means that the fi equations for all curves were reasonable.     Figure 7 shows the water absorption by unit area during the testing time. As i trated in Figure 7, the water absorption by unit area increased linearly with the increa measuring time. Moreover, the water absorption by unit area was decreased by the creasing water-cement ratios (cured in standard curing environment) and the increa CO2 curing time. This was attributed to the fact that the decreasing water-cement r and the increasing CO2 curing time could effectively improve the compactness o cement paste and reduce the number and volume of pores leading eventually to creasing the water absorption by unit area [49]. Furthermore, when cured in the curing environment, the influence of the water-cement ratio on the water absorptio unit area was contrary to the standard curing environment due to the enhanced car ation by the increased water-cement ratio. Table 8 shows the values of fitting param    Figure 7 shows the water absorption by unit area during the testing time. As illustrated in Figure 7, the water absorption by unit area increased linearly with the increasing measuring time. Moreover, the water absorption by unit area was decreased by the decreasing water-cement ratios (cured in standard curing environment) and the increasing CO 2 curing time. This was attributed to the fact that the decreasing water-cement ratios and the increasing CO 2 curing time could effectively improve the compactness of the cement paste and reduce the number and volume of pores leading eventually to decreasing the water absorption by unit area [48]. Furthermore, when cured in the CO 2 curing environment, the influence of the water-cement ratio on the water absorption by unit area was contrary to the standard curing environment due to the enhanced carbonation by the increased water-cement ratio. Table 8 shows the values of fitting parameters for all fitting curves. As demonstrated in Table 8, the fitting degrees are higher than 0.91, which represents the high accuracy of the fitting equation.   Figure 8 shows the mechanical strengths (compressive strength and flex strength) of cement paste. Table 9 shows the fitting results for mechanical strengths depicted in Figure 8 and Table 9, the mechanical strengths increased obviously with curing age, ranging from 1 day to 28 days. Meanwhile, slow increase of mechan strengths occurred with the curing age, increasing from 28 days to 90 days. It can be served from Figure 8 that the mechanical strengths were improved by the CO2 cur especially when the curing age was lower than 28 days. When the specimens were cu in the standard curing environment, the mechanical strengths increased with the creasing water-cement ratio. However, when the CO2 curing was provided, the chanical strengths were improved by CO2 curing, and the higher water-cement r could enhance the effect of improvement. This was attributed to the fact that higher ter-cement ratio led to more pores and loose hydration products, resulting in the creased mechanical strength [50]. Additionally, the CO2 curing could effectively impr the compactness of hydration products, thus, increasing the corresponding mechan strengths [51]. When the CO2 curing was applied, the water-cement ratio was effec for the mechanical strength of cement paste due to the more compact hydration prod by CO2 curing.   Figure 8 shows the mechanical strengths (compressive strength and flexural strength) of cement paste. Table 9 shows the fitting results for mechanical strengths. As depicted in Figure 8 and Table 9, the mechanical strengths increased obviously with the curing age, ranging from 1 day to 28 days. Meanwhile, slow increase of mechanical strengths occurred with the curing age, increasing from 28 days to 90 days. It can be observed from Figure 8 that the mechanical strengths were improved by the CO 2 curing, especially when the curing age was lower than 28 days. When the specimens were cured in the standard curing environment, the mechanical strengths increased with the decreasing water-cement ratio. However, when the CO 2 curing was provided, the mechanical strengths were improved by CO 2 curing, and the higher water-cement ratio could enhance the effect of improvement. This was attributed to the fact that higher water-cement ratio led to more pores and loose hydration products, resulting in the decreased mechanical strength [49]. Additionally, the CO 2 curing could effectively improve the compactness of hydration products, thus, increasing the corresponding mechanical strengths [50]. When the CO 2 curing was applied, the water-cement ratio was effective for the mechanical strength of cement paste due to the more compact hydration products by CO 2 curing.

Micro Analysi
The thermogravimetric (TG) analysis and differential thermal analysis (DTA) were determined to reflect the hydration products of cement. The testing temperature increased from 20 • C to 950 • C. Figure 9 shows the TG/DTA results of cement pastes with watercement ratio of 0.3 cured in CO 2 for 1, 3 and 28 days, respectively. The TG and DTA of all groups can be divided into three steps. The first step was the temperature of 20-150 • C with the mass loss and the thermal entropy losses of the three groups were 0-0.97% and 0-0.37 mW·mg −1 , respectively, which were caused by the evaporation of free water in cement paste. Moreover, when the temperature ranged from 150 • C to 430 • C continuous mass loss and thermal entropy loss occurred. This was attributed to the fact that the water in the C-S-H pores and the decomposition of C-S-H gel consumed by the hydration and carbonation of cement. Furthermore, when the temperature ranged from 430 • C to 600 • C, the mass loss and the thermal entropy loss decreased rapidly with the increasing temperature. This was attributed to the decomposition of Ca(OH) 2 (CH) crystals. When the temperature was in the region of 600-900 • C, there was a mass loss in the TG curve as well as a significant peak in the DTA curve, which corresponds to the decomposition of calcium carbonate from CO 2 curing.
water-cement ratio of 0.3 cured in CO2 for 1, 3 and 28 days, respectively. The TG and DTA of all groups can be divided into three steps. The first step was the temperature of 20-150 °C with the mass loss and the thermal entropy losses of the three groups were 0-0.97% and 0-0.37 mW·mg −1 , respectively, which were caused by the evaporation of free water in cement paste. Moreover, when the temperature ranged from 150 °C to 430 °C continuous mass loss and thermal entropy loss occurred. This was attributed to the fact that the water in the C-S-H pores and the decomposition of C-S-H gel consumed by the hydration and carbonation of cement. Furthermore, when the temperature ranged from 430 °C to 600 °C , the mass loss and the thermal entropy loss decreased rapidly with the increasing temperature. This was attributed to the decomposition of Ca(OH)2(CH) crystals. When the temperature was in the region of 600-900 °C , there was a mass loss in the TG curve as well as a significant peak in the DTA curve, which corresponds to the decomposition of calcium carbonate from CO2 curing.  Figure 10 shows the SEM micrographs of specimens cured in CO2 for 1, 3, and 28 days, respectively. As shown in Figure 10a, it could be seen that the hydration products at 1 day comprised of needle-like C-S-H gel and flake-like C-H crystals with flocculent phases in between, and the microstructure was mainly a porous and weak cross-linked network. After being cured for 3 days, a denser microstructure and formation of calcium carbonate particles could be observed, as shown in Figure 10b,c. With the increasing CO2 curing age, the calcium carbonate particles increased and became compact. This was attributed to the fact that the CO2 curing could accelerate the carbonation of cement, leading to the formation of calcium carbonate. The calcium carbonate formed a dense structure inside the cement paste; therefore, the SEM micrographs became denser with the increasing CO2 curing age, which agreed well with the strength results discussed in the earlier section.  Figure 10 shows the SEM micrographs of specimens cured in CO 2 for 1, 3, and 28 days, respectively. As shown in Figure 10a, it could be seen that the hydration products at 1 day comprised of needle-like C-S-H gel and flake-like C-H crystals with flocculent phases in between, and the microstructure was mainly a porous and weak cross-linked network. After being cured for 3 days, a denser microstructure and formation of calcium carbonate particles could be observed, as shown in Figure 10b,c. With the increasing CO 2 curing age, the calcium carbonate particles increased and became compact. This was attributed to the fact that the CO 2 curing could accelerate the carbonation of cement, leading to the formation of calcium carbonate. The calcium carbonate formed a dense structure inside the cement paste; therefore, the SEM micrographs became denser with the increasing CO 2 curing age, which agreed well with the strength results discussed in the earlier section.

Conclusions
In this study, the influences of CO2 curing on the hydration and mechanical properties of cement pastes with different water-cement ratios were investigated. The following conclusions can be drawn from the above experimental findings: The CO2 curing led to a decreased setting time of cement paste. The relationship between the setting time and the water-cement ratio can be deduced as a linear function. When cured in standard curing environment, higher water-cement ratio demonstrated a positive effect on the setting time; however, when cured in a CO2 curing environment, the effect was the opposite.
The electrical resistivity of cement paste increased with the curing time in a standard curing environment, which followed a cubic function. The carbon dioxide curing facilitated the increase in electrical resistivity by accelerating the carbonation of cement.
Both flexural and compressive strengths of CO2-cured cement pastes were higher than the corresponding standard cured samples; when the water-cement ratio was higher, the enhancing effect was more obvious. The correlations of dry shrinkage rate and mechanical strengths could be expressed through quadratic functions. However, the water absorption by unit area and the testing time conformed to the positive linear function.
TG/DTA analysis and SEM observations concurred that CO2 curing process facilitated the formation of calcium carbonate through the consumption of CH which led to a more densified microstructure. Therefore, the improved mechanical properties can be attributed to a higher degree of cement hydration and a more compact microstructure induced by CO2 curing.

Conclusions
In this study, the influences of CO 2 curing on the hydration and mechanical properties of cement pastes with different water-cement ratios were investigated. The following conclusions can be drawn from the above experimental findings: The CO 2 curing led to a decreased setting time of cement paste. The relationship between the setting time and the water-cement ratio can be deduced as a linear function. When cured in standard curing environment, higher water-cement ratio demonstrated a positive effect on the setting time; however, when cured in a CO 2 curing environment, the effect was the opposite.
The electrical resistivity of cement paste increased with the curing time in a standard curing environment, which followed a cubic function. The carbon dioxide curing facilitated the increase in electrical resistivity by accelerating the carbonation of cement.
Both flexural and compressive strengths of CO 2 -cured cement pastes were higher than the corresponding standard cured samples; when the water-cement ratio was higher, the enhancing effect was more obvious. The correlations of dry shrinkage rate and mechanical strengths could be expressed through quadratic functions. However, the water absorption by unit area and the testing time conformed to the positive linear function.
TG/DTA analysis and SEM observations concurred that CO 2 curing process facilitated the formation of calcium carbonate through the consumption of CH which led to a more densified microstructure. Therefore, the improved mechanical properties can be attributed to a higher degree of cement hydration and a more compact microstructure induced by CO 2 curing.